WPR14LA038

The pilot receiving instruction’s failure to maintain main rotor rpm while practicing manual throttle control and the flight instructor’s delayed remedial action, which resulted in a hard landing.

During the flight, the flight instructor moved the governor switch from the "automatic" to the "manual" position, which disengaged the full authority digital engine control governor, so that the pilot receiving instruction could practice manual throttle control using the twist grip on the collective. This was the first time that the pilot had flown this make and model of helicopter in the manual mode. When the pilot performed left and right 360-degree hovering turns, he found it difficult to rotate the twist grip but was able to manipulate it and maintain main rotor rpm (NR) within normal limits. The instructor then asked the pilot to perform a left, closed traffic pattern back to the departure airport. The pilot and the instructor reported that the takeoff, transition into cruise flight on the downwind leg, and initial descent were uneventful. The pilot reported that the helicopter was on final approach below 400 feet above ground level (agl) when he noticed that the NR was decreasing. He rotated the twist grip to increase power, but the NR continued to decrease. The low NR audio warning then sounded, and the instructor joined the pilot on the controls. The two pilots lowered the collective to enter an autorotation. The pilot said that the NR never recovered to the normal range even though the twist grip was rotated to its maximum (full-open) position. The flight instructor reported that the helicopter was about 200 to 300 feet agl when the NR began to decrease rapidly. The instructor stated that he took the controls and attempted to roll on more throttle but found that the throttle was already fully open. He said that he lowered the collective in an attempt to conserve NR. Both pilots reported that they pulled up on the collective to cushion the landing but that insufficient NR was remaining to slow the descent. The helicopter landed hard, slid forward on the taxiway, and yawed about 90 degrees left. The instructor reported that, when the helicopter came to a stop, the engine was not running, and the pilot reported that he was unaware of the engine's status. Postaccident examination revealed no binding or other anomalies with the operation of the manual throttle control system or the throttle friction control wheel on the right (pilot's side) collective. Neither the helicopter's vehicle engine multifunction display nor its digital engine control unit recorded any faults for the accident flight. The engine was installed in a test cell, started, and tested in both automatic and manual governor modes, and no anomalies or uncommanded shutdowns were experienced, and the engine's performance met the manufacturer's specifications. Given the engine's satisfactory performance during the postaccident test run and the lack of mechanical anomalies with the manual throttle control system, it is likely that the pilot receiving instruction mismanaged the twist grip throttle control during the approach, which led to a decay in NR. The instructor apparently did not notice the decay in NR until the low NR audio warning sounded, at which point, the helicopter was at too low an altitude to correct the situation.

On December 12, 2013, the VEMD was examined at the facilities of American Eurocopter located in Grand Prairie, Texas. The examination confirmed that the data recorded by the VEMD for the accident flight showed no over-limits or failures. A few of the most recent flights before the accident flight were reviewed, and no over-limits were recorded. On December 12, 2013, a download of data recorded by the DECU was conducted at the facilities of Turbomeca USA in Grand Prairie, Texas. No faults were recorded during the accident flight. The download of the DECU matched the results of the VEMD examination. On December 12, 2013, the engine was examined and test run at the facilities of Turbomeca USA. The engine was installed in a test cell, started, and tested in both automatic and manual governor modes. No anomalies or un-commanded shutdowns were experienced, and the engine's performance during all tests conducted met the manufacturer's specifications. Review of photos taken by airport personnel revealed scars in the asphalt surface of taxiway A that led from the helicopter's initial impact point about 80 feet south to its final resting position. The scars at the initial impact point were consistent with the helicopter's skids and tail section contacting the pavement. The scars corresponding to the skids were initially parallel to the centerline of the taxiway before deviating to the left and leading to the helicopter, which was sitting upright on an easterly heading. The helicopter's tail boom was separated from the fuselage and remained attached only by wires. The tail rotor guard was separated, the lower vertical fin was bent to the right, and both tail rotor blades were damaged. There was no apparent damage to the main rotor blades. The helicopter was recovered from the taxiway to the ramp where it was examined on October 30, 2013, by the NTSB investigator-in-charge, an FAA inspector, and a mechanic employed by the operator. The governor switch was found in the "AUTO" position, and the twist grips were found in the full open throttle position with the "FLIGHT" stop on the right side grip disengaged. According to the flight instructor, he moved the governor switch from the "MAN" to the "AUTO" position after the helicopter came to a stop. It is unknown whether the twist grips or "FLIGHT" stop were moved before the examination took place. The engine was visually examined, and no anomalies were noted. The twist grip was rotated through its full range of motion while the operator's mechanic observed the movement of the throttle linkage at the HMU; no binding or other anomalies were noted with the operation of the manual throttle control system. The throttle friction control wheel on the right collective was loosened and tightened, and, as expected, the twist grip became easier and more difficult, respectively, to rotate. The master switch was turned on, and the flight report for the accident flight was reviewed on the Vehicle Engine Multifunction Display (VEMD). According to the VEMD, the flight number was 2644 and the recorded duration of the flight was 21 minutes. This recorded time started when the engine accelerated through 60% Ng, and ended when it decelerated through 10% Ng and the Nr decreased below 70 rpm. There were no over-limits or failures recorded. Following the examination, the engine, the DECU, and the VEMD were removed from the helicopter for further examination and testing. The helicopter was manufactured in 2000 and had accumulated about 2,634 hours at the time of the accident. It was powered by a Turbomeca Arriel 2B engine, which had a single-channel FADEC-type governor with a manual back-up in the form of a twist grip throttle control. The FADEC system included a digital computer or Digital Engine Control Unit (DECU), a pump/metering unit assembly or Hydro-Mechanical Unit (HMU), and electrical/mechanical links between the helicopter's controls, the DECU, and the HMU. The twist grip throttle control had two ranges, one to reduce fuel flow and one to increase fuel flow, that were separated by a disengageable stop, referred to as the "VOL" or "FLIGHT" stop. The "FLIGHT" stop was installed on the right side collective only. When practicing FADEC-governor failure procedures (or in the case of an actual FADEC-governor failure), the stop is manually disengaged by the pilot, allowing the grip to be rotated through its full range of motion. The AS350 B3 Flight Manual Supplement titled "Engine Failures Training Procedures" described the procedure for FADEC-governor failure training. The training procedure said that "in steady flight conditions," the pilot should set the governor switch to the "MAN" position and then follow the emergency procedure in the Flight Manual for a red "GOV" warning light. The AS350 B3 Flight Manual listed the following pilot actions to be performed for a red "GOV" warning light: · Check flight parameters · Maintain NR in green range · Unlock the FLIGHT detent (VOL), the fuel flow can be modified by turning the twist grip: - to the left to increase fuel flow - to the right to decrease fuel flow · Only apply small amplitude adjustments, synchronized with the collective pitch control in order to maintain NR in the green range. · Fly the approach at 40 [knots] and adjust the fuel flow rate to maintain NR within the upper section of the green range. Slowly reduce the speed[;] if necessary adjust the fuel flow rate slightly on the twist grip to maintain NR within the green range. On final approach, when the collective pitch is increased on reaching hover, let the NR drop for touchdown, reduce the fuel flow rate before lowering the collective pitch. With regard to returning from manual mode to automatic mode, the Flight Manual stated that the governor switch "can be replaced in the AUTO position irrespective of the NR value. Then return the twist grip to the FLIGHT detent (VOL)." Eurocopter Information Notice No. 2169-I-67, "Rotor Flight Controls, Use of Twist Grip," dated June 15, 2010, included information about FADEC-governor failure training. Page 6 of the information notice stated, in part (emphasis in original): Training for total governor failure must be carried out with an experienced instructor. The training starts by simulating a total engine governor failure by setting the "Auto/Manu" selector to "Manu." During training, regardless of the maneuvers carried out, it is always possible to de-activate the failure simulation by resetting the selector to "Auto." The automatic governor becomes immediately active provided that the twist grip has not been reduced beyond 30° ("Idle" switch activated). The change to "Manu" mode freezes the fuel flow, lights up the red "GOV" light and activates the GONG (as for an actual total governor failure). "Manu" mode shall be engaged only in stabilized flight conditions. In "Manu" mode, the fuel flow is adjusted by the pilot using the twist grip. The pilot (on the RH side) must disengage the "Flight" stop of the grip. Relative to the frozen fuel flow, the pilot can then increase power by turning the grip in the increase range or reduce power by turning the grip in the reduction range. The pilot must become accustomed to continuously coordinating movements with the collective pitch control lever and twist grip, hence the need to train regularly. He must be accustomed to correctly coordinating actions in flight before carrying out a complete landing. If the pilot does not feel sure of himself, he must not hesitate to return to "Auto" mode and then back to "Manu" mode from stabilized flight to perfect his pitch/grip coordination. Flight control in "Manu" mode must be fully mastered before carrying out a complete landing. According to the limitations section of the AS350 B3 Flight Manual, with power on, the range for NR on the ground at low pitch is 375 to 385 rpm, and the range for NR in stabi